Source:

Response:

Shells on mountains are easily explained by uplift of the land.
Although this process is slow, it is observed happening today, and it
accounts not only for the seashells on mountains but also for the other
geological and paleontological features of those mountains.
The sea once did cover the areas where the fossils are found, but they
were not mountains at the time; they were shallow seas.

A flood cannot explain the presence of marine shells on mountains for
the following reasons:

Floods erode mountains and deposit their sediments in valleys.

In many cases, the fossils are in the same positions as they grow in
life, not scattered as if they were redeposited by a flood. This
was noted as early as the sixteenth century by Leonardo da Vinci
(Gould 1998).

Other evidence, such as fossilized tracks and burrows of marine
organisms, show that the region was once under the sea. Seashells
are not found in sediments that were not formerly covered by sea.

References:

Gould, Stephen J., 1998. The upwardly mobile fossils of Leonardo's
living earth. In: Leonardo's Mountain of Clams and the Diet of
Worms, New York: Three Rivers Press, pp. 17-44.